Protective system



May 1963 T. F. BELLINGER PROTECTIVE SYSTEM Filed Aug. 2, 1961 MOTORNETWORK Y A L E On .wwzoammm SLIP SLIP AND CURRENT 3,tl9tl,$%PR'llTECTlVlE SYSTEM Thaddeus F. llellinger, West Allis, Wis, assignorto Allis- Chalmers Manufacturing tlompany, Milwaukee, Wis. Filed Aug. 2,1961, SEE. No. 128,832 4 Claims. (rill. did-17h) This invention relatesgenerally to controls for synchronous motors. This invention relatesmore specifically to a protective system that deenergizes a synchronousmotor if the motor fails to accelerate properly to synchronous speed orif the motor pulls out of synchronism.

The protective device of this invention is intended for a synchronousmotor of the type that has a stationary polyphase armature winding and adirect current excited field winding that rotates at synchronous speedwith the ampere turns of the armature winding. A synchronous motor ofthis type will not operate at less than synchronous speed without someadditional means. Usually the rotor of a synchronous motor has anauxiliary windin (a squirrel cage winding) that cooperates with thearmature winding to form an induction motor for accelerating the motorto synchronous speed. The squirrel cage winding ordinarily has onlysufficient thermal capacity to accelerate the motor under normalconditions and the motor will not operate safely as an induction motorfor more than a very short time. If for some reason the motor does notaccelerate to synchronous speed properly, the armature should bedeenergized to prevent overheating the motor. A system that monitors thetemperature of the squirrel cage winding and deenergizes the armaturewinding as the emperature approaches an unsafe limit is called asquirrel cage protective system.

The field winding of a synchronous motor provides a useful but quiteround about measure of the temperature of the squirrel cage winding. Theexciter is not connected to energize the field winding until the motorhas accelerated as an induction motor to nearly synchronous speed. Whilethe motor is accelerating, a voltage is developed in the field winding.This voltage depends partly on transformer action and partly ongenerator action, and the magnitude of this voltage is a rather complexfunction of the speed of the rotor. The frequency of the voltagedeveloped in the field winding is called the slip frequency. The slipfrequency equals the supply system frequency at starting and approacheszero as the motor accelerates to synchronous speed.

The slip frequency, and to a lesser extent the voltage magnitude, areimportant indicators of the power that goes into heating the squirrelcage winding, and the rate that heat is removed by cooling. At hi h slipfrequency (eg. when starting) the current in the squirrel cage windingis very high, and the squirrel cage winding heats rapidly. At low slipfrequency (near synchronous speeds) the current is appreciably lower,and at synchronous speed the current in the squirrel cage windings iszero. The motor usually has a fan mounted on the rotor, and the fancools the motor more effectively as the speed increases. The differencebetwen the total heat receiver and the total heat lost by the squirrelcage winding determines the temperature of the squirrel cage winding.Thus, any device that integrates the slip frequency indicatesqualitatively the temperature of the squirrel cage winding.

The slip frequency integrator must follow the heating of the motor veryaccurately. If the integrator is inaccurate, it will underprotect oroverprotect the motor in different operating regions of the motor.Overprotection wastes the costly built-in reserve of the motor, and itmay cause the motor to trip out unnecessarily and thereby interruptimportant services. Underprotection may allow the motor to be damaged.

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The thermal time delay relay is quite often used to integrate the slipof a synchronous motor, and it illustrates the desirable features of aslip integrator and the problems in obtaining these features. Aninductor that is connected in parallel with the electrical heatingelement of the relay provides a voltage across the heating element whichis proportional to the slip frequency. The thermm relay is anapproximate thermal model of the squirrel cage winding and it heats atabout the same rate as the motor. One of the advantages of a thermalrelay is that it cools at the same rate that it heats, and it accountsfor the temperature of the motor from a recent start. However, thethermal relay has several shortcomings in a protective system for asquirrel cage winding. A thermal relay responds rather slowly to theheating current, regardless of the magnitude of the current.Commercially available relays do not have sufficiently fast response toprotect a synchronous motor at maximum slip frequency where the motorcan operate safely for only a few seconds. In addition, the thermalrelay continues to react to its accumulated heat after the motor haspassed through a potentially dangerous operating range, and these relaysoccasionally trip out the motor when in fact the potential danger to themotor did not materialize. This effect is called overshoot. Several wellknown time delay relays have fast response without overshoot.Unfortunately, there is no necessary relation between the suitability ofthe relay to respond fast without overshoot and the further requirementthat the relay react to the voltage of the field winding in the same waythat the squirrel cage winding heats. In fact, some of the mostpromising integrating relays are apparently incompatable with the signalthat the field winding provides. The protective system of this inventionovercomes this problem.

The protective system of this invention includes a time delay relay thathas a high damping factor to prevent overshoot and has fast response forprotection at high slip speeds. The system also has an electricalnetwork that receives the signal of the field winding and produces amodified signal that excites the relay to follow the heating of thesquirrel cage winding. Preferably, the network is adjustable so thatstandardized relays can be applied easily to motors that have differingthermal characteristics.

One object of this invention is to provide a new and improved protectivedevice for a motor with a squirrel cage Winding.

Another object of this invention is to provide a new and improvedsquirrel cage protective device that includes a relay with a highdamping factor and with fast response.

Another object of this invention is to provide a simple and convenientelectrical network that adapts the mechanical and electricalcharacteristics of a time delay relay to the thermal and electricalcharacteristics of a motor with a squirrel cage winding.

Another object of this invention is to provide a new and improvedprotective system that is adjustable to match the characteristics of thestandardized relays to motors of differing characteristics.

The drawin and the detailed description of the invention will suggestother objects and advantages.

FIG. 1 is a schematic drawing of a motor and the protective system ofthis invention;

FIG. 2 is a graph showing characteristics of the motor and of componentsof the protective system of FIG. 1;

FIG. 3 is a detail schematic drawing of a component of the protectivesystem;

FIG. 4 is a graph showing characteristics of the motor and a componentof the protective system; and

FIG. 5 is another embodiment of the component of HG. 3.

accuse-s FIG. 1 shows a generalized synchronous motor 10, a specificrelay 12, and a generalized network 14 that couples the relay 12 to themotor It The network 14 has input terminals 15 connected to receive asignal from the motor 1% and it has output terminals 16 that areconnected to transmit this signal in a modified form to the relay 12.The relay l2 and the network 14 make up the protective system of thisinvention. The motor it has an armature winding 13, a field windirn l9,and a squirrel cage winding 29. A switch .5 connects the armaturewinding 18 to a polyphase alternatin current system 24. A switchoperator 26 opens and closes the switch 2-4 to control the motor id. Therelay 12 is connected by conductors 2.7 to open the switch 23 when therelay 12 closes. During starting the armature winding 18 cooperates withthe squirrel cage winding 2%) to form an induction motor. While themotor it) is accelerating to synchronous speed, the field winding 1%produces an electrical signal that contains information about theheating rate of the motor. Slip rings 3%, connect the field winding toan exciter (not shown) when the motor has accelerated to nearlysynchronous speed. During acceleration, the slip rings 3% connect thefield winding 19 through a switch 31 to the network 14-.

The relay 12 includes a magnetic coil 32, a pivotable armature 33, aframe 34 and a viscous damped time delay element 36. A full waverectifier 37 connects the coil 32 to be energized through the network14. The rectiher 37 prevents the relay 12 from opening during currentzeros at low slip frequency. One end of the armature 33 is pivotallyconnected tothe frame 34. The armature 33 and the frame 54 carrycooperating contacts 3?. A spring 39 normally holds the armature 33 andthe contacts 38 open.

The time delay element 36 comprises a nonmagnetic closed tube 42, amagnetic plunger 43 positioned inside the tube, oil 44 which slows themovement of the plunger in the tube, and a spring 45 which tends to holdthe plunger in the position of FIG. 1. The coil 32 is positioned on thetube 42 to produce a magnetic flux in the armature 33, the frame 34, theplunger 43, and the air gap between the armature and the plunger.

Plunger 43 is in the starting position of FIG. 1 when the switch 23 isfirst closed, and the large air gap between the plunger and the armature33 prevents the coil 32 from closing the armature. During the timedelay, the coil operates only on the plunger 43 inside the oil filledtube 4-2, and the plunger 43 moves toward the armature as the coil isenergized. As the plunger 4-3 nears the armature 33, the magnetic fieldincreases, and at a selected position of the plunger the armature swingsclosed. When the armature 33 closes, the contacts 338 complete thecircuit 27 to the switch operator 26', and the switch operator opens theswitch 23 and deenergizes the motor 16.

As is well known, the relay 12 has an inverse time delay characteristic.It requires a shorter time to close if more current is supplied to thecoil 32. Since the relay is intended to follow the motor heating, thevelocity of the plunger 43 as a function of current more directlyrelates the relay characteristics to the problem of the protectivesystem than does the more commonly used inverse time characteristic.

Since the velocity of the plunger 43 in the oil filled tube 42 is afunction of the current in the coil 32, the position of the plungerindicates the integration of the current in the coil 32 with respect totime. Thus, if the current that the field winding 19 supplies to thecoil 32 is proportional to the heating rate of the motor, the coil wouldadvance the time delay element 36 at a rate corresponding to motorheating, and the position of the plunger would indicate the heataccumulated by the motor It) during starting. In fact, the electricalsignal at the slip rings 30 does not suitably relate the electrical andmechanical characteristics of the relay 12 to the thermal and electicalchar rcteristics of the motor 1%). The network 14 provides this link. Asalready mentioned, the motor heating rate is nonlinear with respect toslip frequency. The velocity of the plunger 43 on the other hand is anearly linear function of current in the coil 32, within the range ofFIG. 2. FIG. 2 illustrates these two characteristics in a commoncoordinate system. As FIG. 2 shows, the problem of the network 14 is totransform the slip frequency signal at the slip rings to a currentsignal having the shape of the motor heating characteristic. The relayresponse (specifically the velocity of the plunger 43) to current andthe motor response (heating) to slip are drawn to different scales thatare arbitrarily selected so that the characteristics intersect at thepoint 44- of maximum slip. Since the relay responds nearly linearally tocurrent, as FIG. 2 shows, the current output at the terminals 16 of thenetwork 4 indicates the relay response to the signal at the slip rings3%. Thus the response of the network 14 indicates the response of therelay 12.

FIG. 3 shows one specific network 1 2a comprising a capacitor and aresistor 47 connected in series, and a variable resistor 45 connected inparallel with the series resistor and capacitor combination. All or aportion of the resistor 47 and a portion of the resistor 48 representsthe resistance of the coil 32. The resistor 45 is at least partia ly adiscrete resistance, and it is preferably variable. Thus, the network 1m and the voltage at the slip rings establish the magnitude of thecurrent in coil 32 and thereby establish the force on the plunger FIG. 4shows the effect on the network response of the capacitor :6 alone(curve A), the capacitor 46 and resistor 47 alone (curve B), andcombination of the capacitor 46 and the resistors 47, 48 (curveNetwork). In the example of FIGS. 3 and 4, the network in; accuratelyfollows the motor heating at starting and at the point 49 nearsynchronous speed where the switch 31 opens and the motor it) beginssynchronous operation. in between these points the network Ma and t rerelay l2 closely but not exactly follows the motor heating.

As FIG. 4 shows, the capacitor 46 and resistor 47 cooperate to establishthe shape of the curve and the resistor cooperates with the resistor 47and capacitor 46 combination to move the characteristic of the network14:: along the response axis. It is highly desirable to use a variableresistor as at least a portion of the resistor The network 14a can thenbe made of standardized components and the variable resistor 53 can beadjusted to adapt the relay characteristics to motors having a widerange of characteristics.

5 shows a more general example 14b, of the network 14. The network 1412of FIG. 5 has a capacitor 46b and two resistors 47b, that are similar intheir circuit arrangement to the network Mia but have somewhat differentvalues than the corresponding components of the network 14b. Inaddition, the network 1412 has an inductor in series with the resistor43.

The inductor 5t tends to eliminate the effect of the resistor at highslip frequency. Consequently, the capacitor as!) may have somewhathigher capacitance than the corresponding capacitor es of the network14:: and still provide the same current at starting at and nearsynchronous speed. The values of the network are selected to provide thesame current at starting as the network Ma, and the network 14b providessomewhat higher current throughout most of the motor operating range andthus more closely follows the motor thermal speed characteristics.

So far F168. 2 and 4 have been described as though the magnitude of thevoltage across the slip rings 3 is constant as the motor it}accelerates. In fact, the voltage magnitude is relatively constant overthe range in which the motor .ril acts as an induction motor and theanalysis thus far of FIGS. 2 and 4 is satisfactory for many protectivesystems. lowever, it is preferable that the network lid link the closingcharacteristics of the relay 1?. to both the frequency and the mavnitudecharacteristics of the signal at the slip rings The relation between thevoltage magnitude and the motor speed cannot be easily generalized asthe relation between the slip frequency and the motor speed has been. inFIG. 4 the response of the network 14 is the response to both thevoltage magnitude and the frequency, but for generality the horizontalaxis is expressed only as slip. For any particular motor there is adefinite voltage magnitude for each value of slip frequency. Thus,voltage magnitude is a function of slip frequency. One technique forconstructing the network response curve of FIG. 4- is to solve thecircuit for the response to selected frequencies and to the voltagemagnitude that occurs at these frequencies.

When the switch 25 is closed, the armature winding 18 and the squirrelcage winding 2t) cooperate to accelcrate the motor. During accelerationthe squirrel cage winding heats appreciably, although not necessarilydangerously. The field Winding l9 energizes the network lid, and thenetwork 14 produces a current that varies in magnitude with the heatingrate of the motor. The current in the coil 32 gives the plunger 43 avelocity that is closely proportional to the current and thereby closelyproportional to the heating rate of the motor. The plunger 43 movestoward the closing point of the relay 1?. as the temperature of themotor it? approaches the acceptable temperature limit. In the usualsituation the motor to reaches nearly synchronous speed before the motorheats dangerously and before the relay l2 closes. A control called thesynchronizing relay (not shown) opens the switch 31 to isolate theprotective system from the field winding and the control connects thefield winding 19 to its exciter for synchronous operation. The spring 45then returns the plunger 43 slowly to the position of FIG. 1.

If the motor is stopped and then restarted before the plunger hasreturned, the relay would operate in the way just described except thatplunger would start from a position that corresponds to heat stillretained by the motor from the previous start. in addition, eachoperation heats the oil 4 and thereby temporarily reduces its viscosity.Thus, the relay l2 closes faster if the motor is still heated from aprevious start.

If the motor It} fails to accelerate properly, the field winding 19 andthe network 14- supply a high current to the coil 3?. long enough forthe relay 1?; to close. At very low speeds, the relay closes within afew seconds. When the relay l2 closes, it completes the circuit of theconductors 27, and the switch operator 26 opens the switch 23 to preventfurther improper operation of the motor lit.

The oil damped relay l2 is a very significant element in the protectivesystem. However, other relays may be more or less suitable. The mostimportant requirements of such a relay are that the relay has goodresponse to high input signals (i.e. in the right hand region of FIG. 2)that the relay has a high damping factor to prevent overshoot, and thatthe relay takes into consideration the temperature of the motor that isdue to a previous start.

The protective system is particularly intended for a synchronous motorbecause the field winding conveniently provides a slip frequency signal.The system is also suitable for an induction motor when a suitable meanssuch as a tachometer provides a signal having a predetermined magnitudeand frequency relation to the motor speed.

In the specific protective system that has been described, the fieldwinding 19, the network 14, and vari- 65 one other impedanccs actsomewhat as a current source. Those skilled in the art will recognizethat the network 1 5 and th field winding 19 can also be connected anddescribed as a voltage source.

Those skilled in the art will recognize other variations in theprotective system, and the claims are intended to cover variationswithin the spirit of the invention.

Having now particularly described and ascertained the nature of my saidinvention and the manner in which it is to be performed, I declare thatwhat I claim is:

1. A protective system for a synchronous motor having a field windingand having a starting means that is operable safely for a limited timethat is a function of motor speed, comprising a relay having an inversetime delay characteristic and being operable to deenergize the motorafter a time delay that is an inverse function of the energization ofsaid relay, and an electrical network for coupling said relay to beenergized by the field winding of the synchronous motor during starting,said notwork comprising the parallel combination of a capacitor and aresistor, said parallel combination connected to adapt the time delaycharacteristics of the relay to the voltage and frequencycharacteristics of the field winding for causing said relay to followthe heating of the starting means.

2. A protective system for a motor having a squirrel cage winding andhaving means producing an electrical signal indicating by its frequencyand magnitude the speed of the motor, said system comprising a timedelay relay having a predetermined closing response to an electricalsignal, means coupling said relay to receive the speed indicatingsignal, said means comprising a capacitor and a resistor connected inparallel to modify the speed indicating signal to match the response ofsaid relay to the heating characteristics of the motor to close therelay when the motor approaches a dangerous temperature condition, andmeans connecting said relay to deenergize the motor when the relaycloses.

3. A protective system for a synchronous motor having a field windingfor operation at synchronous speed and having a squirrel cage windingfor operation at subsynchronous speed, comprising a relay having aviscous damped magnetic time delay element and a coil advancing saidtime delay element at a rate that varies with the energization of saidcoil, and an electrical network including the parallel combination of acapacitor and a discrete resistance, said network coupling said coil tothe field winding and cooperating with the field winding to form asource of current for energizing said coil to advance said time delayelement according to the heating rate of the squirrel cage winding, andmeans connecting the relay to deenergize the synchronous motor at theend of the time delay.

4. A protective system for the starting winding of a synchronous motorhaving a field winding energized by direct current when the motor is insynchronous operation, comprising a relay having a viscous dampedmagnetic time delay element and a coil advancing said element towardclosing at a rate that is established by the current in said coil, anelectrical network coupling said coil to the field winding duringstarting, said network comprising a capacitor and a resistor connectedin parallel and cooperating with the field winding to provide a currentfor said coil which is proportional to the heating rate of the startingwinding, said resistor being variable over a range to adapt the responseof said relay to synchronous motors that heat differently in response tomotor speed, and means connecting said relay to deenergize thesynchronous motor when the time delay element has advanced to close therelay.

No references cited.

1. A PROTECTIVE SYSTEM FOR A SYNCHRONOUS MOTOR HAVING A FIELD WINDINGAND HAVING A STARTING MEANS THAT IS OPERABLE SAFELY FOR A LIMITED TIMETHAT IS A FUNCTION OF MOTOR SPEED, COMPRISING A RELAY HAVING AN INVERSETIME DELAY CHARACTERISTIC AND BEING IPERABLE TO DEENERGIZ E THE MOTORAFTER A TIME DELAY THAT IS AN INVERSE FUNCTION OF THE ENERGIZATION OFSAID RELAY, AND AN ELECTRICAL NETWORK FOR COUPLING SAID RELAY TO BEENERGIZED BY THE FIELD WINDING OF THE SYNCHRONOUS MOTOR DURING STARTING,SAID NETWORK COMPRISING THE PARALLEL COMBINATION OF A CAPACITOR AND ARESISTOR, SAID PARALLEL COMBINATION CONNECTED TO ADAPT THE TIME DELAYCHARACTERISTICS OF THE RELAY TO THE VOLTAGE AND FREQUENCYCHARACTERISTICS OF THE FIELD WINDING FOR CAUSING SAID RELAY TO FOLLOWTHE HEATING OF THE STARTING MEANS.